Recently, many attentions have been given to nanofiltration membranes in the field of water purification technology which essentially requires complete removal of dye materials from industrial waste water or securement of stable supply of public drinking water using, as crude water, surface water or groundwater from which agricultural chemicals or other organic contaminants are to be removed. Such nanofiltration membranes are those positioned in the middle of reverse osmosis membranes and ultrafiltration membranes based on the classification according to pore sizes. Such nanofiltration membranes are driven under a lower pressure condition as compared to a reverse osmosis membrane process and allow filtration of a part of salts, including organic materials. Thus, a nanofiltration membrane process is also referred to as a low-pressure reverse osmosis membrane process in its nature. In other words, since the purity of water produced after filtration does not show a fineness corresponding to ultrapure water required for a semiconductor process or pharmaceutical industry, nanofiltration membranes are used for some applications not requiring a high-efficiency reverse osmosis process obstinately.
Active studies have been conducted about development of nanofiltration membranes having excellent water permeability and a high salt rejection ratio to meet such applications. The inventors of the present disclosure have already developed a polyamide-based composite membrane having a salt rejection ratio required for the field of nanofiltration membranes or reverse osmosis membranes and high water permeability at the level of nanofiltration, and have registered it as patent. Such a composite membrane may increase throughput per unit time and provide increased efficiency during a water treatment process, resulting in high cost efficiency. However, due to the characteristics of such a polyamide-based composite membrane, it shows low chlorine resistance and is susceptible to fouling. Therefore, actual application of the polyamide-based composite membrane to large-scale water treatment industry is limited (Patent Document 1).
In addition, to solve the problem of the polyamide-based composite membrane having low chemical stability, such as chlorine resistance, some studies have been conducted about a nanofiltration composite membrane including hydrophilic alcohol having a chemically stable C—C single bond backbone as an active layer. However, the polyvinyl alcohol active layer has a relatively large thickness and an inadequate crosslinking degree, resulting in a relatively low water permeation flux and salt rejection ratio (Non-Patent Document 1).
Meanwhile, to solve the above-mentioned problems, there has been developed a single crosslinked sodium alginate/polyvinyl alcohol composite membrane for nanofiltration having improved membrane density through the crosslinking with glutaraldehyde alone. However, in this case, it is required to pretreat the polysulfone support used in the composite membrane with a monomer material, piperazine and trimesoyl chloride, in order to reduce the pore size of the polysulfone support and to improve hydrophilicity. In addition, such a composite membrane is not optimized for improvement of a water permeation flux simultaneously with a salt rejection ratio (Non-Patent Document 2).
Therefore, the inventors of the present disclosure have conducted many studies and prepared a double crosslinked sodium alginate/polyvinyl alcohol composite membrane by coating a porous polymer support, such as polysulfone, with a sodium alginate/polyvinyl alcohol blend without a pretreatment process of the porous polymer support and then carrying out crosslinking by using two types of crosslinking agents having a different crosslinking mechanism to improve membrane density. We have found that the composite membrane shows excellent chemical stability, such as chlorine resistance, and has an increased water permeation flux as well as an increased salt rejection ratio, when applied to nanofiltration. The present disclosure is based on this finding.